Current industry practice for evaluating ultrabarrier performance against water vapor transmission is to use a method that measures actual water transmission. These methods include coulometry (U.S. Pat. No. 7,569,128, MOCON, Inc.), detection of radioactive HTO (U.S. Pat. No. 7,257,990, General Atomics) and optical transmission through calcium (U.S. Pat. No. 7,117,720, Philips Electronics). All three of these approaches have severe limitations. They are all water based, and therefore inherently slow. The time required for establishment of steady state diffusion through a membrane is 5 to 10 times ts, where ts=I2/6D. I is the thickness of the membrane, and D is the diffusivity of the permeant. In the case of water diffusing through PET, D is ˜4×10−9 cm2/s at 25 C, so that for a 5 mil (0.005 inch) thick membrane. The minimum measurement time is 10 to 20 hours. In practice, even longer measurement times are required, because the tests involve vapor transmission rates at or near the limit of sensitivity for each technique. Therefore, significant averaging is required to compensate for the poor signal to noise ratio. As a result, it takes on the order of 1 week to be confident that the transmission rate of an ultrabarrier is at or below the detection limit of either a coloumbic detection system or a radioactive tritium measurement. It takes even longer for the calcium test, because it takes a week or more for the measurement cell to achieve internal equilibrium and start to exhibit a steady state transmission rate.
In addition to their slow measurement rates, the above techniques are also limited in their sensitivity. According to product literature from MOCON® Minneapolis, Minn., the limit to their measurement technique is 5×10−4 g/m2/day. The limit of the calcium test is an order of magnitude lower: 5×10−5 g/m2/day. The requirement for an ultrabarrier layer for CIGS solar cells or OLED display devices is ˜10−6 g/m2/day, making the tritium test, having a reported detection limit of 2×10−7 g/m2/day, the only technique sufficiently sensitive to verify performance at the required level. Even this test is not really sensitive enough, however, as efficient statistical sampling methods for process control require a continuous measure of the process, both in the out-of-limits and within-limits regimes. The HTO test could only provide such a measure for permeation rates that are just within the control limits, and in most cases could only provide a “pass/fail” type of response. The statistical requirements on “pass/fail” type measurements are so burdensome as to render process control essentially impossible.
The solution to the first limitation of these methods, the long measurement time, is to use an alternative test gas as a proxy for the water. Helium, for example, has a diffusivity in PET that is 500 times larger than that of water. As a result, ts for helium diffusing through a 5 mil membrane is about 15 seconds, resulting in a measurement time on the order of minutes. Such a substitution is a valid probe of the barrier properties, because permeation through an ultrabarrier occurs via micropores in the inorganic layer, rather than by permeation of the inorganic material itself. Once a correlation has been established between water vapor transmission and helium permeation for the bare PET (or other polymer) substrate, a rapid helium-based test can give a reliable value for the water vapor transmission rate (WVTR).
The required sensitivity for a test and measurement system based on the helium proxy principle is straightforward to estimate. The noise floor of the calcium test represents a reduction in WVTR over the bare substrate by a factor of 40000. From the permeation rate of helium through PET (˜2×10−8 scc/cm/s/atm), and assuming a 10 cm diameter membrane and a driving pressure differential of 1 atm, this 40000 reduction factor gives a helium transmission rate of 3×10−9 scc/s, well within the range of commercial helium mass spectrometer systems. To be useful as a process control technique, however, requires a sensitivity two orders of magnitude below that. Fortunately, state of the art, oil-free helium mass spectrometers are specified to have a sensitivity of 5×10−12 scc/s which is three orders of magnitude below the calcium test limit.
Yilvisaker (U.S. Pat. No. 5,361,625) describes passage of gas through a membrane by exposing one side of the film to a test gas and exposing the other side of the film to a carrier gas.
In practice, however, measurements are limited by the background helium signal due to permeation through elastomer seals. Therefore, there is a need for an apparatus designed to hold ultrabarrier membranes for testing, while reducing or eliminating this background signal.